1. Field of the Invention
This invention relates to an electro-optical waveguide element provided with an optical waveguide, formed on a substrate having electro-optical effects, and at least a pair of electrodes positioned adjacent to this optical-waveguide, in which the modulation or switching of guided light is carried out by applying a voltage between the electrodes.
2. Description of the Prior Art
As is disclosed in, for example, Japanese Unexamined Patent Publication No. Hei-2(1990)-931, an electro-optical waveguide element has already been known which comprises a thin-film optical waveguide formed on a substrate having electro-optical effects, a grating-shaped electrode (hereinafter referred to as an EOG electrode) that forms electro-optical diffraction gratings in the optical waveguide, and a driver circuit for applying a voltage to this EOG electrode. In this electro-optical waveguide, guided light which travels through the optical waveguide is selectively diffracted in accordance with a state of application of the voltage to the EOG electrode.
The use of such an electro-optical waveguide element makes it possible to modulate light to be used in response to the presence or absence of diffraction or the degree of diffraction when either diffracted light or non-diffracted light (zero-order light) is used. In addition, an optical switch can be constituted which switches an optical path of guided light in response to the presence or absence of diffraction.
Moreover, as is disclosed in, for example, JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 20, No. 4, April, 1981, pp. 733-pp. 737, an electro-optical waveguide element is also known. This electro-optical waveguide element is made up of two optical channel waveguides, formed on a substrate having electro-optical effects, that constitute a directional coupler, and a planar electrode provided on each optical channel waveguide. Guided light that travels through either one of the optical channel waveguides is selectively shifted to the other optical channel waveguide in response to the state of the application of a voltage to the planar electrode.
The use of such an electro-optical waveguide element makes it possible to use the light emitted from the other optical channel waveguide, and to modulate the light in response to the state of application of a voltage to the electrode. Further, an optical switch can be constituted which switches an optical path of guided light.
In the electro-optical waveguide elements set forth above, it is necessary to form an optical buffer layer between the electrode and the substrate in order to prevent the scattering or absorption of light caused by the electrode. This buffer layer has been conventionally formed of SiO.sub.2 or A1.sub.2 O.sub.3.
It is acknowledged that the electro-optical waveguide element having the previously mentioned buffer layer is susceptible to so-called DC drift phenomena, i.e. the phenomena in which an operating point varies with the application of a voltage.
To prevent such DC drift phenomena, as is disclosed in the previously mentioned JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 20, No. 4, April, 1981, pp. 733-pp. 737, a proposal is conventionally put forward in which an area between the buffer layer and the electrode is removed, and the buffer layer is separated for each electrode.
However, if input light has a short wavelength or a high intensity, a leakage current occurring in the optical waveguide will become larger, and therefore it is impossible to sufficiently reduce the extent of the DC drift, even by separating the buffer layer for each electrode in the manner as previously mentioned.